The present invention relates to a method and apparatus for manufacturing a laminated stator core for a motor or other dynamoelectric machine, and in particular to an improved method and apparatus for aligning the laminations of the core so that the rotor accommodating bore is perpendicular to the plane defined by the bottom of the stack.
In the manufacture of dynamoelectric machines, such as electric motors, the stator generally comprises a core comprising a plurality of stacked laminations secured together by welding or bonding. In certain motor applications, such as hermetic refrigeration compressors, it is customary to mount the bottom of the stator to a plurality of mounting pads or surfaces machined on the inner surface of the compressor housing. The stator is connected to the housing generally by means of a plurality of bolts which extend through the stator core and are threadedly secured to the compressor housing. This results in a cantilever mounting arrangement for the stator in that only one end thereof is fastened to the compressor housing and the other end extends upwardly. It is desirable that the plane defined by the machined surfaces of the compressor housing be perpendicular to the axis of the rotor when the motor is installed. In order to accomplish this, it is necessary that the rotor accommodating bore of the stator core be perpendicular to the lower surface of the core itself, since it is this surface which is supported on the machined surfaces of the compressor housing.
It is important for the air gap between the stator and rotor of the motor to be very uniform along the entire axial length of the motor. In many hermetic compressor applications, the rotor is supported by a bearing only at one end thereof, often the same end at which the stator is connected to the compressor housing. Because of the cantilevered supporting arrangement for the rotor, although it is relatively easy to maintain an accurate air gap at the end thereof nearest the bearing, normal flexing and deflection of the rotor at the opposite end will result in a wider variance in air gap, taking into consideration normal machining and bearing tolerances. Accordingly, in order to minimize as much as possible the error in air gap at the end of the stator core furthest from the mounting surfaces of the compressor, it is necessary that the rotor accommodating bore be very accurately aligned perpendicularly to the reference plane, especially at the distal end thereof.
In one prior art technique for manufacturing a stator core, the pre-measured stack of loose laminations is placed on a pallet having an accurately machined upper surface and an opening extending therethrough, wherein the opening is of approximately the same diameter as the rotor accommodating bore of the stacked laminations. The pallet and lamination stack are placed over an alignment post that is secured to a tooling plate, wherein the post is mounted in such a fashion that it is perpendicular to the tooling plate with as much accuracy as is possible. The lower surface of the pallet is machined very accurately to be parallel with the upper surface thereof so that, when the pallet and lamination stack are placed over the post and supported on the tooling plate, the post causes the laminations to align themselves in such a manner that the rotor accommodating bore is perpendicular to the reference plane defined by the upper surface of the pallet.
The pallet is then clamped to the tooling plate, a plurality of bolts are inserted through clearance holes in the stacked laminations and are simultaneously threaded into tapped holes in the pallet by means of power operated nut drivers. When the bolts are tightened thereby compressing the laminations, the compressed laminations may exert a bias on the pallet so that when the clamping force is removed, one corner of the pallet may be lifted away from the tooling surface. When this distortion of the pallet occurs, the rotor accommodating bore of the lamination stack will no longer be perpendicular to the reference plane, and it will be necessary for the machine operator to straighten it by manual techniques. The amount of distortion of the pallet can be measured by air gauges located under the four corners of the pallet. After the compressed stack of laminations has been aligned, the core is dipped into an adhesive, such as a suitable epoxy, the core is spun to distribute the epoxy and eliminate the excess, the bore is burnished, and the mounting bolts are removed. The bonded stator is then transferred to the next stage of the manufacturing process, such as the insertion of the stator windings.
Certain elements of the prior art process and apparatus described above are disclosed in U.S. Pat. No. 3,490,143, issued to Bobbie B. Hull, which patent is expressly incorporated herein by reference.
One problem with the stator core manufacturing technique outlined above results from the fact that the alignment pin is cantilevered to the tooling plate in that it is fastened thereto at only one end. Although the pin is very rigid at the bottom so that perpendicularity with the tooling plate can be obtained with a high degree of accuracy, it is much less rigid at its distal end and is able to be deflected because the distal end is not supported. Furthermore, any non-perpendicularity of the pin is amplified to a greater extent at the distal end thereof thereby resulting in a larger deviation from the true centerline at the distal end.
Although it is very difficult to ever obtain a condition of precise perpendicularity of the alignment pin, the fact that the pin is supported by one end as the laminations are bolted down, results in a greater misalignment at the upper end of the lamination stack with a resulting greater error in air gap tolerance. This result is particularly troublesome in the manufacture of stator cores for compressors wherein the rotor is often supported by a bearing at the lower end of the stator core rather than at the upper end so that the misalignment of the rotor is likely to be greater at the upper end thereby resulting in a larger air gap error at this location. Since the misalignment of the rotor accommodating bore manufactured by the method outlined above is also greater at the upper end than at the lower end of the stator, the air gap error is even more pronounced.
The problem to be solved, then, is to align the stator core laminations in such a manner that very accurate perpendicularity of the rotor accommodating bore is achieved, not only at the bottom of the stack as can be accomplished with the above-discussed prior art technique, but also at the top of the stack where misalignment can least be tolerated.
It is desirable to be able to check the misalignment of the rotor accommodating bore prior to the bonding of the laminations. Once the laminations are bonded, they cannot be broken apart, and in many cases the bore cannot be remachined in the event that it is too far out of tolerance. It is a practice, in some applications, to insert the winding into the loosely stacked laminations before bonding in order to decrease in-process inventory, and also because of the greater ease in inserting the windings into loose laminations. Furthermore, the bonded cores can sometimes be deflected slightly by the winding insertion machinery. In the case where the cores are bonded with the windings in place, an even greater expense results in the event that the bonded assembly has to be scrapped if a misalignment in the stator bore is not detected prior to bonding. In view of the above, it is desirable for the degree of misalignment of the rotor accommodating bore to be measured prior to the step of bonding the laminations together.